1. Field of the Invention
The present invention relates to measurement systems for measuring the width of a gap such as may be useful in focusing systems for a scanned laser lithography system or the like.
2. Description of the Related Art
The present invention relates to an apparatus for measurement of small gaps. The described device has particular application in photolithography systems for semiconductor production where a gap may exist between a work piece and an optical lens system or the like. It should be noted that the present invention has application in other fields than semiconductor processing. For example, the apparatus of the present invention may be useful in measurement of gaps between a lens system of microscope and a target work surface. In the photolithography field a number of devices for measurement of gaps are known.
A first known measurement device is described with reference to FIG. 1. The system may be described as a grazing incident optical technique apparatus in which a light source 104 is utilized to direct a light beam at work surface 102. The beam is directed at a point on the work surface beneath the optical path of a stepper lens assembly 101. The light beam is reflected to a light detector 103 and the total length of the light beam path may be determined. A calculation of the gap between the stepper lens assembly 101 and the work surface 102 may be completed based on the calculation of the length of the light beam path. Thus, this system allows measurement of the gaps between the work surface 102 and the stepper lens 101 over the same area as is targeted for printing. As will be seen, the feature of measurement of the gap over the same area as is targeted for printing is an important aspect of the present invention. However, this described prior art system illustrated by FIG. 1 may not work on all surfaces. For example, this system may not work on a surface which is not adequately reflective.
A second prior art system for measurement of gaps is illustrated with reference to FIG. 2. In FIG. 2, a capacitance gage 204 is provided to measure the capacitance level between the gage 204 and work surface 202. The distance between stepper lens 201 and work surface 202 may then be calculated based on this capacitance level. However, as can be seen with reference to FIG. 7, a typical work surface 702 may not have a uniform surface. Therefore, any measurement means, such as measurement means 721 located to measure the gap between such measurement means 721 and the work surface 702, may result in an incorrect measurement if the distance measured is not the distance directly below the lens 701.
FIG. 3 illustrates a variation of the system of FIG. 2 in which air gage jets 304 are located off the axis of stepper lens 301 to measure the gap between the air gage jets 304 and 305 and the work surface 302. This type of system relates to the preferred embodiment of the present invention in its use of air gage jets which are utilized to measure the back pressure produced resistance from work surface 302; however, the system does not allow for measurement directly over the work surface which is targeted for printing and, therefore, is subject to the short comings illustrated by FIG. 7.
It is therefore an object of the present invention to develop a measurement means for measuring a gap between a work surface and a targeting device in which the measurement is taken over the surface which is targeted.
It is further an objective of the present invention to develop such a measurements means which is capable of working in conjunction with virtually any surface including both reflective or non-reflective surfaces.
The preferred embodiment of the present invention utilizes an air gage as a measurement means. A number of such gages are known in the art. One such gage is described by U.S. Pat. No. 4,142,401 Wilson. This reference describes a gage for measuring a width of a gap. The type of gage described by Wilson may be utilized for example in the measurement system of FIG. 3. As can be appreciated from a review of Wilson, this gage allows measurement with use of relatively low pressurization.